This application relates to imaging. More specifically, this application relates to controlling light propagation through scattered media for imaging applications.
Certain imaging applications, such as biomedical imaging applications, require that light propagation through scattering media at high speeds be controlled. As light propagates through such media, and particularly through biological tissue, it becomes increasingly scattered, thus limiting the optical imaging depth to depths on the order of 1 mm. Wavefront control techniques have recently been introduced that allow for focusing through turbid media. These techniques rely on the deterministic nature of scattering processes to shape the incident wavefront to compensate for the scattered photons. Iterative methods divide the light incident on a scattering sample into N spatial input modes. The optimal phase of each mode is measured and set to create a focus on the opposing side of the scattering material. Other iterative techniques optimize the input modes in parallel, thus increasing the speed at which the focus is formed.
Another technique measures the transmission matrix through the scattering material. With the transmission matrix, the relationship between the input modes and output modes through the sample is understood quantitatively and phase masks can be calculated that focus to any mode in the output plane. Other techniques apply a phase conjugation to a recorded scattered field for focusing through turbid media.
Techniques that rely on the deterministic nature of multiple scattering to shape the incident wavefront and to pre-compensate for the scattering effects of light propagation encounter difficulties in certain media, notably in living biological materials. The imaging depth into biological materials is limited by scattering, and living biological materials have speckle decorrelation times on the millisecond timescale. This fast rate of change limits the value of various methods of focusing through turbid media, making them too slow because of measurement-rate limitations from the wavefront modulation device. Such methods typically use liquid-crystal spatial light modulators (“LC-SLM”) for phase-only wavefront modulation, which is more efficient for creating a focus than amplitude-only modulation. the LC-SLMs' switching speed is limited by the rate at which the liquid crystals can align in the device, typically on the order of 10s of Hz and much slower than the kHz rate needed for the millisecond timescale of biological tissue.
There is accordingly a need in the art for improved methods and systems for wavefront optimization.